1. Field of the Invention
The present invention relates to radiographic image conversion panels and radiographic image detectors used in the formation of radiographic images.
2. Description of the Related Art
Radiographic images such as X-ray images have been widely used in medical diagnosis of disease conditions. In particular, radiographic images based on intensifying screen-film combinations have undergone enhancements in terms of sensitivity and image quality during a long history and consequently remain in use in the medical field worldwide as the imaging system with high reliability and excellent cost performance. However, this image information is analogue and thus cannot be processed freely or transmitted instantaneously in contrast to currently developing digital image information.
Recently, digital radiographic image detectors such as computed radiography (CR) systems and flat panel detectors (FPDs) have come in use. These radiographic image detectors directly give digital radiographic images and allow the images to be directly displayed on displays such as cathode ray tube panels and liquid crystal panels. Thus, there is no need for the images to be created on photographic films. Consequently, the digital X-ray image detectors have decreased a need for the image formation by silver halide photography and have significantly enhanced diagnostic convenience at hospitals and clinics.
The computed radiography (CR) is one of the digital X-ray image techniques currently used in medical practice. However, CR X-ray images are less sharp and are insufficient in spatial resolution as compared to screen film system images such as by silver halide photography, and the level of their image quality compares unfavorably to the quality level of screen film system images. Thus, new digital X-ray image techniques, for example, flat panel detectors (FPDs) involving thin film transistors (TFTs) have been developed (see, for example, Non Patent Documents 1 and 2).
In principle, a FPD converts X-rays into visible light. For this purpose, a scintillator panel is used which has a phosphor (scintillator) layer made of an X-ray phosphor that, when illuminated with X-rays, convert the radiations into visible light that is emitted. In X-ray photography using a low-dose X-ray source, it is necessary to use a scintillator panel with high luminous efficiency (X-ray to visible light conversion) in order to enhance the ratio (the SN ratio) of signal to noise detected from the scintillator panel. In general, the luminous efficiency of scintillator panels is determined by the thickness of the scintillator layer (the phosphor layer) and the X-ray absorption coefficient of the phosphor. The light produced in the phosphor layer upon illumination with X-rays is scattered more markedly in the scintillator layer with increasing thickness of the phosphor layer, and consequently the sharpness of X-ray images obtained via the scintillator panel is lowered. Thus, setting of the sharpness required for the quality of X-ray images automatically determines the critical thickness of the phosphor layer in the scintillator panel.
Further, the shape of phosphor constituting a phosphor layer is also important in order for scintillator panels to exhibit high brightness and excellent sharpness. In many scintillator panels, scintillator layers are composed of columnar crystal phosphors which are usually disposed on substrates such as supports. The columnar crystals constituting scintillator layers extend perpendicularly to the substrates such as supports, in order to be able to efficiently emit fluorescence produced therein in a direction perpendicular to the substrates such as supports. With this configuration, the scintillator panels ensure brightness and sharpness as well as achieve strength in the direction perpendicular to the substrates such as supports (hereinafter, this direction will be also referred to as the “film thickness direction”).
Various studies and attempts have been made focusing on the shapes of phosphor crystals that form scintillator layers. For example, Patent Document 1 directed to obtaining a scintillator panel having high brightness and excellent sharpness discloses a radiographic conversion panel having on a substrate a phosphor layer which includes columnar crystals containing a phosphor base material and having a specific shape. The phosphor layer in the radiographic conversion panel of Patent Document 1 has a combination of a first phosphor layer with a specific film thickness including a phosphor base material and a second phosphor layer containing the phosphor base material and an activator. Patent Document 1 is based on the finding that excellent sharpness is obtained when the columnar phosphor crystals forming the phosphor layer satisfy a specific ratio of the crystal diameter at the outermost surface to the crystal diameter at 10 μm height from the substrate side.